CM870 loosing coolant/possible inflame

[Coronado126]

An internal coolant leak is a very possible issue, and if you've been having to add coolant and have no leaks on the ground, then you have to start suspecting internal issues.

You're mechanic is correct about the coolant being able to escape when the valves start opening. When that happens the piston can then force the coolant out of the cylinder.

If you have the tooling available, you can rotate the engine yourself by hand with a ratchet on the drivers side of the engine to verify if has an internal failure. If it turns very hard by hand then you need to repair the engine. Keep in mind there will be "tough spots" when you rotate an engine because you'll fight compression but that will fade away and go back to turning smoothly.

https://youtu.be/SOx1AxRSP8E

Here's one of Rawze's videos on engine timing, you'll see he has a big ratchet in the turning tool location in the front cover, which is what I'm talking about using.

Don't be afraid to get a second opinion from a different shop/mechanic. Overhauls are not cheap, and some shops get different price breaks from the OEM than others.